1. Field of the Invention
The present invention relates to digital signal processing (DSP), and more specifically to the implementation of a low-pass filter for filtering undesired high frequency components from a signal.
2. Related Art
Low pass filters are often used for eliminating high frequency spectral components from different types of signals. For example, as is well known in the relevant arts, low pass filters may be used in modems to eliminate (or substantially attenuate) spectral components having a frequency greater than 3.4 kHz in an analog signal. Such analog signals are generally generated at another modem (source modem) to encode digital data. A receiving modem may receive the analog signal and filter high frequency spectral components to recover the digital data as described below.
To recover the digital data from a received analog signal, a modem often needs to filter high frequency signal components (over 3.4 kHz bandwidth) typically present in a received analog signal. Ideally, only the frequency components having less than 3.4 kHz bandwidth should be passed, and the higher frequency components should be eliminated completely.
However, for many practical applications substantial attenuation (e.g., 60 dB) of the undesired components is sufficient. Typically, the transition from zero attenuation to 60 dB attenuation in 1 kHz presents complex challenges as is well known in the relevant arts.
Low pass filters are often implemented using digital signal processing (DSP) circuits as the circuits provide a cost-effective, compact (i.e., less space consuming), and flexible (i.e., designer can implement desired computational steps) way of eliminating the undesired frequency components. In a typical configuration using a DSP circuit, an analog to digital converter (ADC) may sample an analog signal to generate a high frequency data stream.
The ADC may be implemented using sigma-delta modulation, and generate a high frequency single bit output. A low pass filter implemented as a DSP circuit may process the digital samples to eliminate the high frequency components. The encoded digital data may be recovered from the digital samples representing the filtered signal.
A prior filtering system may be implemented using non-recursive polyphase filters (also known as finite impulse response filters). The output of a finite impulse response (FIR) filter always returns to zero when excited with a unit impulse. However, as is well known in the relevant arts, the FIR filter may need to perform a large number of computations to perform the filtering operation.
The large number of computations may be problematic in some situations as the filtering operation may need to be performed in "real time," that is, as the analog signal is being received. By filtering in real-time, large buffers may be avoided. The time required for computations may be decreased by employing complex hardware, but the required number of gates ("gate count") may be too numerous for integration on a single integrated circuit. In addition, circuits generally require more electrical power for performing a large number of computations, and accordingly such solutions may not be suitable at least in portable devices, which are often powered by electrical sources storing a limited amount of electrical power.
Therefore, what is needed is a circuit which optimizes one or more of the gate count, the number of computations, cost, and/or electrical power required while performing a filtering operation.